The Optimization of Severe Shot Peening Coverage for Enhanced Fatigue Performance of AISI 4140 Steel: A Combined Experimental and FE-Cell Approach

Shot peening is widely used to improve fatigue performance by introducing compressive residual stresses, but the influence of very high coverage levels on medium carbon steels such as AISI 4140 remains unclear. This work investigates conventional and severe shot peening at 100%, 400%, 1000%, and 1500% coverage under a fixed 18A Almen intensity, combining rotating-bending fatigue experiments with a finite element cell (FE-Cell) approach for residual stress quantification. Fatigue tests were conducted at stress amplitudes of 37 MPa, 40.5 MPa, and 44 MPa, supported by surface roughness, hardness, and fractographic characterization. The results show a strong coverage–stress interaction: at 37 MPa, severe shot peening at 1000% coverage yields the maximum fatigue life improvement, whereas at 44 MPa the optimum shifts to 400% coverage, and excessive coverage (1500%) produces over-peening, microcrack networks, and reduced life. FE-Cell simulations reveal that increasing coverage deepens and intensifies compressive residual stresses but also promotes stress redistribution at extreme coverage. The combined findings define an optimal processing window of 400–1000% coverage for AISI 4140, balancing deep compressive residual stresses with controlled surface integrity and providing practical guidelines for industrial severe shot peening of rotating components.